1. Field of the Invention
The present invention relates to a light source used in a display device, a sensor, a scanner, a printer and the like, and particularly relates to a light source used as an illuminating means of a liquid crystal display device and to a liquid crystal display device using these light sources.
2. Description of Related Art
As an illuminating means of a liquid crystal display device, several methods using a tube-shaped lamp such as a cold cathode ray tube or an LED (light emitting diode) as a primary light source (light source emitting light by itself) are in use. In case of combining a liquid crystal panel of a transmission type with a tube-shaped lamp, as shown in FIG. 2, a structure in which a tube-shaped lamp 21 covered with a reflector 22 is arranged at one side of a planar light guide 24 and a reflecting plate 23 is arranged at the back of the planar light guide 24, and a diffusing plate 25 is arranged at the entrance face side is made to be the mainstream from the viewpoint of space saving. And in case of combining a liquid crystal panel of a reflection type with a tube-shaped lamp, a structure in which a tube-shaped lamp covered with a reflector is arranged at one side of a planar light guide and the planar light guide is arranged at the observer side of the liquid crystal panel of a reflection type is formed.
The use of an LED as a primary light source is superior to a tube-shaped lamp from the viewpoint of power saving and space saving. However, it is difficult to obtain a uniform light emission as a planar light guide since the light emitting face of an LED is small.
Several methods for solving this problem are known. For example, as shown in FIG. 3 disclosed in Japanese Patent Laid-Open Publication No. 10-260,405 (reference 1), a uniform light emission is obtained by arranging a linear light guide 32 for spreading light from an LED 31 in the shape of a line between the LED 31 and a planar light guide 24 (FIG. 3). And as disclosed in Japanese Patent Laid-Open Publication No. 11-231,320 (reference 2), there is also a method of disposing a linear light guide and forming an array of projections on the entrance face of a planar light guide. Further, as a method of using no linear light guide, there is mentioned a method of providing a blind hole 41 depressed in the direction of thickness having a function of refracting and reflecting light emitted from a light source in a light guide member, as disclosed in FIG. 4 in Japanese Patent Laid-Open Publication No.2001-23,423 (reference 3).
By using these methods, it is possible to obtain uniform light emission, so that the planar light guide can be combined with a liquid crystal panel as an illuminating means in a similar manner to the case of a tube-shaped lamp.
In the above-mentioned techniques, the angular distribution of emergent light is so wide that a large amount of the light is not used efficiently. For example, a white LED of chip type used in a liquid crystal display device for a mobile information terminal emits light with distribution of about xc2x160xc2x0, which is much wider than the field of view of an observer. In case that light 51 having such a distribution is incident on a linear light guide or a planar light guide as disclosed in reference 1 or 2 (FIG. 5(a)), the light outputted from the planar light guide has a similar angular distribution (emergent light angular distribution) to the incident light (using such a projection structure as shown in reference 2 makes the emergent light angular distribution more spread). Further, since light from a liquid crystal display panel to an observer 56 has also a similar angular distribution, the amount of light incident on an eye of the observer becomes small (FIG. 5(b)).
In short, the efficiency of utilizing light is lowered. This becomes a problem in particular in a liquid crystal display device for a mobile information terminal in which space and power saving are demanded. In FIG. 5(a), symbol 32 is a linear light guide, symbol 52 is emergent light from the linear light guide, symbol 53 is a liquid crystal panel of a reflection type, symbol 54 is emergent light from a planar light guide, symbol 55 is reflected light for displaying on the liquid crystal panel of a reflection type, symbol 56 is an observer, and symbol 57 is light to come into an eye of the observer.
As a method for solving this problem, there is a method mentioned in Japanese Patent Laid-Open Publication No.2000-315,413 (reference 4). In reference 4 (FIG. 6), a collimating structure covered with a light reflecting plate 61 is shown. The structure has a part that is wide at the light guide side and narrow at the light source side. However, it is required to form a number of said complicated collimating structures between the light source and the light guide. In FIG. 6, symbol 62 is incident light from the light source and symbol 63 is emergent light from the collimating means.
As a method for narrowing the emergent light angular distribution, there is a method of affixing a lens to an LED. However, a light source is required to be far smaller than the lens (the light source is at least {fraction (1/10)} relative to the lens) (FIG. 7(a)). For example, in case that a light source 31 and a lens 71 are close in size as shown in FIG. 7(b), light goes through with various angles, so that a large part of light 72 cannot be used efficiently. Therefore, the method of affixing a lens to an LED requires a lens to be very large in comparison with the size of an LED and is difficult to be used as an illuminating means of a liquid crystal display device from the viewpoint of space saving.
As a method of using a lens shape, it is difficult also to apply a structure of combining a lens face 84 with a total reflection surface 83 (FIG. 8(a)) disclosed in xe2x80x9ca paper by Singo Tamai et al., pp.1,247 in a collection of papers read at the Electronics Society Convention 2000, the Institute of Electronics, Information and Communication Engineers of Japanxe2x80x9d (reference 5) to an illuminating means of a liquid crystal display device (FIG. 8(b)), since the combined structure needs a much larger lens shape in comparison with the size of an LED in the same way as described above.
Although it is conceivable also to use an LED disclosed in reference 5 as a primary light source, it is difficult from the viewpoint of space saving to be used as a primary light source, because the LED disclosed in reference 5 is much larger than an LED chip due to the size of a lens and a structure of a reflecting plate 82. Further, such a complicated structure may cause an increase in cost.
The purpose of this invention is to provide a light source that is simple and space-saving, and has a narrow light distribution angle.
A light source unit of the present invention is characterized by a light director taking in light from a primary light source through an entrance face and outputting it, having reflection faces on both sides of the entrance face. The light director preferably has two refractive index interfaces on the way from the entrance face to the exit face, wherein the refractive index of the entrance face side is larger than that of the exit face side in the first refractive index interface and the refractive index of the exit face side is larger than that of the entrance face side in the second refractive index interface.
It is possible to arrange the angular distribution of incident light with high efficiency by reflecting the light being large in angle of incidence from the primary light source by using the reflecting surfaces, and making the reflected light enter again the first refractive index interface as making smaller its angle of incidence onto the first refractive index interface by means of the reflecting surfaces at both sides of the entrance face, and make narrow the angular distribution of emergent light with high efficiency by further making narrower the emergent light angular distribution by means of the second refractive index interface.
On the assumption that a straight line passing through the middle point of the entrance face and the middle point of the exit face is the principal axis, the refractive index of the entrance face side of the first refractive index interface is n1, the refractive index of the exit face side of the first refractive index interface is n2, a desired emergent light angular distribution value having the principal axis as its center is xcex2 (xcex2 is an absolute value), and an angle made by the principal axis and the first refractive index interface is xcex81, it is possible to obtain a desired emergent light angular distribution (xcex2) with high efficiency by forming the first refractive index interface out of two inclined planes meeting the expression (1).
90xc2x0xe2x88x92sinxe2x88x921(n2/n1)xe2x89xa6xcex81xe2x89xa690xc2x0xe2x88x92{sinxe2x88x921(n2/n1)xe2x88x92xcex2}xe2x80x83xe2x80x83(1)
It is possible to make the light reflected by the first refractive index interface enter again the first refractive index interface with high efficiency as making narrow the angle of incidence onto the first refractive index interface by a fact that an angle xcfx86 made by the reflection faces at both sides of the entrance face and the principal axis meets the following expression (2).
xcex81xe2x88x9245xc2x0xe2x89xa6xcfx86xe2x89xa6xcex81xe2x80x83xe2x80x83(2)
Further, on the assumption that an angle made by the second refractive index interface and the principal axis is xcex82, in case of forming the second refractive index interface out of two inclined planes meeting the following expression (3), it is possible to make an emergent light angular distribution narrow with high efficiency.
xcex81/2xe2x89xa6xcex82xe2x80x83xe2x80x83(3)
As a primary light source for these light sources it is preferable to use an LED (light emitting diode) from the viewpoint of space saving and power saving and the like.
And one aspect of the present invention is a linear light source provided with a linear light guide in the direction of emergent light of said light director exit face. It is possible to obtain emergent light with high efficiency by providing reflection faces on the opposite faces to a linear exit face in this linear light guide. And the opposite faces to the exit face may be inclined to the exit face. Further, it is possible to suppress leakage light and obtain emergent light with high efficiency by touching closely or adhering the exit face of the light director and the entrance face of the linear light guide to each other. And it is possible also to form the light director and the linear light guide into one body by assuming the straight line tying to each other thexe2x80x94intersecting points of the second refractive index interface and the side faces. Further, it is possible to suppress leakage light and obtain emergent light with high efficiency by covering the other faces than the exit face and said light director or linear light source with a reflective member.
Another aspect of the present invention is to provide a planar light guide in the direction of emergent light of said linear light source exit face. It is possible to obtain emergent light with high efficiency by providing projections on the faces opposite to the exit face of a planar light guide. Further, it is possible to obtain uniform emergent light by making the refractive index of the projections on the opposite faces to the exit face larger than the propagating part. It is possible to suppress leakage light and obtain emergent light with high efficiency by touching closely or adhering the exit face of the linear light source and the entrance face of the planar light guide to each other. The linear light source and the planar light guide may be formed into one body.
It is possible to achieve a bright liquid crystal display device by mounting said planar light guide on a liquid crystal display panel. By making a planar light guide serve also as a substrate forming a liquid crystal display panel and providing a transparent layer being smaller in refractive index than the planar light guide at the liquid crystal layer side of the planar light guide, it is possible to suppress a depth feeling to occur at the time of mounting the liquid crystal display panel with the planar light guide, further secure the amount of guided light propagating through the planar light guide and obtain a uniform display.